Polymers (polyparabanic acid) (hereinafter sometimes described as "PPA") having an imidazolidinetrione ring (parabane ring), which are obtained by hydrolysis of polymers obtained by reaction of hydrogen cyanide with one or two or more diisocyanates, are already known, and this PPA has attracted special interest as heat resistant polymer lately.
Specifically speaking, PPA may be prepared, for example, by the process disclosed in Japanese Patent Publn. No. 20960/1974 or the like.
Since the polymer obtained by the process disclosed in the patent publication cited above and having structural units represented by the following formula (a) has a parabane ring in its main skeleton, said polymer is basically excellent in various characteristics such as mechanical strength, heat resistance and solvent resistance. ##STR1##
However, on investigation on this polymer, the present inventor has found that said polymer having structural units represented by the above-mentioned formula (a) markedly varies in its characteristics according to the kind of the substituent attaching to the nitrogen atom in said structural unit. That is, the characteristics of the polymer such as mechanical strength, heat resistance and solvent resistance markedly vary according to the kind of substituent R in the recurring unit represented by the following formula (b). ##STR2##
Particularly, on closer investigation on characteristics of the polymers disclosed concretely in the above-cited patent publication, it has been found that said polymers are relatively low in mechanical strength.
As a result of investigation on the relationship between the kind of the above-mentioned substituent R and the characteristics of the polymer having said substituent R, the present inventor has found that when the substituent R of the polymer having the above-mentioned recurring unit (b) is a specific aromatic group, the resulting polymer is able to have characteristics such as mechanical strength superior to those of the polymer concretely disclosed in the above-cited patent publication, and has obtained such technical information that this improvement is the above-mentioned characteristics can be made without a large sacrifice of other characteristics. Furthermore, the present inventor has obtained such a technical information that the characteristics of the polymer having the specific aromatic group mentioned above may be improved particularly when said polymer is formed into a copolymer containing a recurring unit having other specific substituent in addition to the recurring unit having the above-mentioned specific aromatic group. Thus, the present inventor has attained the present invention.